The surface stabilized ferroelectric liquid crystal (SSFLC) light valve has been shown to possess properties useful in a number of electrc-optic device applications requiring high contrast ratio. These include electro-optic shutters, spatial light modulators for optoelectronic computing, and flat panel display devices. The physics and operation of the surface stabilized FLC has been extensively described elsewhere (Clark, N. A. et al. (1983) Mol. Cryst. Liq. Cryst. 94:213). U.S. Pat. Nos. 4,367,924 and 4,563,059 of Clark and Lagerwall describe SSFLC cells. In such FLC cells, an appropriate FLC material is incorporated between glass plates, the inner surfaces of which are coated with a transparent electrode. Application of an electric field to the electrode induces a switching or rotation of the molecular orientation of the FLC materials in the cell. The application of the electric field, thus, switches between two orientations of the optic axis of the cell each of which represents a different transmission state of the cell. The speed of response (switching speed) is often important. This response speed is given approximately by equation 1: ##EQU1## where .tau. is the optical response (10%-90%) to an applied voltage step of magnitude E, .eta. is the orientational viscosity, and P is the ferroelectric polarization density. In the surface stabilized state, FLC molecules lie in smectic layers perpendicular to the glass plates (the so-called bookshelf geometry). The FLC optic axis makes an angle .+-..THETA. with respect to the layer normal. Application of an electric field across the glass plates of the cell allows the optic axis of the cell to be rotated through 2.THETA.. The tilt angle is an intrinsic property of the FLC material. SSFLC cells have for the most part been demonstrated with smectic C* materials. However, any chiral tilted smectic LC materials are useful. The voltage requirements for SSFLC switching devices are modest (.+-.10 V), and power consumption is quite low because the FLC switching energy is small. An additional advantage for some applications is that the devices are bistable (Clark, N. A. and Lagerwall, S.T. (1980) Appl. Phys. Lett. 36:899), allowing relatively easy implementation of passive matrix addressing in 1D and 2D arrays.
The contrast (ratio of transmitted light intensity through the cell in the bright and dark states) in a standard SSFLC cell is greatest when the tilt angle .THETA. of the FLC material is 22.5.degree.. Under these conditions, at the half wave thickness (where d=.lambda./2 .DELTA.n) between crossed polarizers (an entrance polarizer and an exit polarizer or analyzer), the dark state will leave the plane of polarization of the input light unchanged, while the bright state will rotate the plane of polarization of the input light through 90.degree.(4.THETA.). In general, in the on (switched) state the plane of polarization of the input light will be rotated through 4.THETA., where .THETA. is the tilt angle.
The orientation viscosity, .eta., generally increases with increasing tilt angle. Often, .eta. increases with tilt angle faster than P, and thus materials with low tilt angle (i.e. .THETA.&lt;15.degree.) often show improved electro-optic response speed relative to similar materials with 22.5.degree. tilt. However this increase in speed is achieved only at the expense of contrast, since the output light in the SSFLC is then rotated through &lt;90.degree., and when crossed polarizers are employed a significant amount of the light in the on state is extinguished at the analyzer.
Sub-millisecond switching times can be achieved at room temperature with SSFLC cells. These switching devices are not hampered by small entrance apertures, as FLC cells can be fabricated on large substrates. In fact, current applications of FLC's include flat panel displays (Inove, H. et al., Int. Display Conference, Oct. 4-6, 1988).
An electroclinic effect has been described with chiral smectic liquid crystal materials (Garoff and Meyer (1977) Phys. L. Rev. Lett. 38:848 and (1979) Phys. Rev. A 19:338). Light valves based upon this electroclinic effect have been described. For example, Andersson et al. (1987) Appl. Phys. Lett. 51:640 described a "soft-mode" switching effect and soft mode cells, incorporating smectic A* LCs, having the same bookshelf geometry as described for SSFLC cells. The rotation of the optic axis of these cells is dependent on the strength of the applied field. Electroclinic effect based cells have been demonstrated for chiral smectic A*LC materials. These cells exhibit several attractive features, including fast response and voltage regulated true analog response, affording, e.g. true analog gray scale intensity modulation. However, for all materials known, the maximum tilt angle achieved due to the electroclinic effect is small (i.e. .THETA.&lt;15.degree.), and the maximum contrast ratio that can be achieved in such devices is low for the same reason as for low-tilt SSFLC cells. Similar to smectic C*LC devices, when the field induced tilt angle is large, switching speeds are relatively slow, while low-tilt materials give, in general, faster switching speeds. Thus, in the smectic A*LC devices the same speed for contrast tradeoff is generally required as for smectic C* devices. Electroclinic effect based cells have many of the characteristics of SSFLC cells.
Thus, there are two important light-modulation technologies involving chiral smectic liquid crystals (S*LCs). These are the smectic C*LC based SSFLC light valves, and the smectic A*LC based electroclinic light valves. Some illustrative applications of devices based upon these technologies are as follows. Smectic C*LC applications: Single element modulators with large aperture and fast switching speed; 1D and 2D arrays of modulators (or pixels). These can be addressed using active matrix techniques (for example thin film transistor arrays or more conventional integrated circuit back-planes) where the fast switching speed and high interaction strength are important, or addressed using passive matrix techniques, where the fast switching speed and bistability are important. Focussing upon the 2D arrays, these could be flat panel computer displays. It seems clear that, while active matrix addressing of nematic LC panels can lead to good gray-scale (color) panels, it is very difficult to manufacture the required active matrix back planes.
Recently the difficulty in manufacturing active matrix displays has prompted the flat panel computer display industry to consider an alternative approach to true gray scale, that is spatial averaging or halftone gray scale in much more easily manufactured passively addressed displays to achieve the required quality (Pleshko (1990) Information Display 6:10-11). In this type of application the high contrast, bistability and high resolution of smectic C*LC devices are advantageous over the conventional nematic LC devices. Indeed, the speed of FLCs allows the possibility of additional gray levels in the device by temporal averaging as well as spatial averaging. Even so, with the switching speeds currently possible with FLCs, it is not possible to achieve the number of gray levels and frame rates necessary to create passively addressed full motion, full color video panels.
Smectic A*LC applications: The smectic A*LC modulators provide the same switching speed advantages as smectic C*LC modulators, but give voltage-regulated analog gray scale response instead of bistable 1 bit response. A smectic A*LC would thus be useful in single element and array modulators where true analog gray scale is required. However, improved switching speeds and temperature stability are required. Thus, when the smectic A*LC devices are very fast, the interaction strength is low, when the devices show strong interaction (i.e. high voltage-induced tilt angle), the response speed is slower, and the temperature must be carefully controlled.
Bradshaw et al., International Patent application WO 87/06021 refer to a liquid crystal display arrangement which provides very low minimum light transmission, useful as a shutter and for high contrast digital displays. A single cell device comprising a liquid crystal material having a cholesteric phase above a chiral smectic and also having a large cholesteric pitch is described. It is indicated that these properties of the FLC material are necessary to provide a uniform alignment of the FLC layers in the cell and thus result in cells with improved contrast.
Coulson et al. International Patent application WO 87/06020 refer to devices using ferroelectric smectic liquid crystal materials and to a cell wall surface treatment to give high tilt to contacting liquid crystal molecules. This surface treatment is said to provide improved uniformity of the cells by improving alignment of the FLC molecules in the cell. The application refers to the use of FLC materials having a cholesteric phase at higher temperature to the chiral smectic phase and preferably having a high cholesteric pitch. The surface treatment is said to provide a surface tilt of above 5.degree.. A surface treatment including a process of oblique evaporation of silicon monoxide is specifically described.
Handschy et al. (1987) Optics Lett. 12:611 refer to the use of FLC elements or cells in optical parallel logic gates. The reference describes the "cascading" of two FLC cells to create a logic gate. Each of the SSFLC cells employed had a tilt angle of 22.5.degree. or more.